This invention relates to semiconductor devices and more particularly to solid state transferred electron devices.
Transferred electron devices are well known and are characterized by a device comprised of semiconductor material such as gallium arsenide (GaAs), a group III-V compound which has two valleys in its conduction band. When a high electric field is applied across such an element, electrons are transferred from the lower energy satellite valley to the higher energy satellite valley and because mobility of electrons in the higher energy valley is less than the mobility in the lower energy valley, the average speed of electrons decreases with an increase in electric field. When the intensity of the internal electric field applied from an outside source exceeds a critical value, a high field domain is created near the cathode, which is thereafter translated to the anode by the action of the applied electric field. When the high field domain reaches the anode, it disappears and an impulsive current is caused to flow through the semiconductor substance because of disappearance of the high field domain. Following this disappearance, a new high field domain is created near the cathode and the same sequence is repeated at a frequency determined by the length of the drift region over which the domain travels. For a more detailed treatment of the subject, reference may be made to chapter 11 entitled, "Transferred-Electron Devices" of the text Physics of Semiconductor Devices, 2nd edition, 1981 by Simon Sze, at pages 637-678, and published by Wiley-Interscience.
In a conventional n+-n-n+, transferred electron oscillator, which is also known as a Gunn diode, the voltage drop occurs primarily over the n region which is the drift region. In an effort to improve device performance, injection limited cathode contacts have been used instead of n+ ohmic contacts in order to provide a more uniform electric field. One known type of injection limited contact is a Schottky barrier, while the other is a two-zone cathode structure and is disclosed, for example, at pp. 667-670 of the aforementioned Physics of Semiconductor Devices. Notwithstanding the attempt to overcome the non-uniformity of the electric field in transferred electron devices, any variation in the doping of the drift region also alters the electric field causing degraded performance and reduction in the radio frequency (rf) conversion efficiency. Accordingly the difficulty of achieving abrupt doping profiles in conventional Gunn diodes leads to irreproducible and non-otpimized devices.
It is also known that Schottky barriers exhibit several inherent limitations, one being, for example, for a particular metal-semiconductor system, the barrier heights are virtually constant and operational stability is related to the metallurgy of the contact system. Furthermore, interface states in interfacial layers play a dominant role in determining the Schottky barrier transport properties which can lead to undesirable hysteresis effects, particularly in metal-GaAs structures. Attempts have been made in the past to modify the heights of Schottky barriers, however, such devices still exhibit relatively large barrier heights which necessitate the use of relatively high power.
Accordingly, it is an object of the present invention to provide an improvement in transferred electron devices.
Another object of the present invention is to provide a transferred electron oscillator having an improved electron injecting means.
A further object of the present invention is to provide an improved transferred electron oscillator having precise field and potential profiles.
And it is still a further object of the present invention to provide an improved transferred electron device including a planar doped barrier having a relatively low barrier height which is controllable.